Record blocking in the northern hemisphere has meant outstanding cold for this time of year across Europe, North America, even down to Cancun Mexico (which recorded an all time December low of 51F a couple weeks ago). The negative North Atlantic Oscillation (NAO) and negative Arctic Oscillation (AO) has been a persistent and dominate feature so far this winter, and the results of it can be seen globally. Closer to home, we’ve experienced a primarily cold and dry December, with the only heavy precipitation events coming in liquid form. However, that may change during the Christmas Eve-Day period, if everything falls into place correctly. Let’s examine what needs to stack in our favor in order for a White Christmas in the tristate area.

We know we have a negative NAO and AO in place, that can be evidenced by the massive west based blocking across north-central Canada right now.

A major difference between this upcoming potential event and all others so far this season is the fact that we’re about to see a spike in the PNA, meaning the first semblance of a significant Western ridge we’ve seen all winter. Strong La Nina’s are very skimpy on providing us with PNA ridges, thus it’s important that we utilize this window if you’re an east coast snow lover. PNA spikes and negative, west based NAO’s tend to be a common denominator of 10″+ snows in the I-95 corridor of DCA-BOS.

Note the following image. The 18z GFS H5 (500mb) pattern valid for Christmas Day afternoon. There are couple of things which I believe we need to key in on here.

1)We’ve got the impressive amplitude of the Western ridge extending all the way up to northwest Canada. More importantly, the axis of it will be positioned east of the classi Boise ID location, more towards central Montana.

2) Consequently, the 18z GFS idea of closing off the H5 low in the Appalachains/Ohio Valley seems far-fetched to say the least. Considering wavelengths, a ridge axis in UT-MT would argue for the downstream short wave to amplify and tilt negative further SE.

3) In addition, the Polar Vortex sitting east of Hudson’s Bay, coupled with the high heights to its west, should ensure that this is NOT a warm Lakes Cutter or inland runner. AKA rain event for the east coast.

4) Strong La Nina climatology argues for a more progressive, faster, more suppressed solution, even with the strong blocking we have in place.

So what will happen? The chances of this being a warm, rain event are very slim due to the reasons outlined above. However, unfortunately I cannot rule out a suppressed/out to sea solution for the same reasons. It’s possible the energy of the southern and northern jet streams congeals/phases a bit further east than currently depicted, meaning the H5 low would close off far to our SE and produce little snow.

With that being said, this pattern is clearly the most favorable we’ve seen all winter so far, and probably the best we can get in a strong La Nina season. The PNA ridge is crucial as it will aid in forcing the northern stream southward to get involved, phasing with the sub tropical jet short wave, hence the storm. Therefore, I’d rate the chance of a white Christmas to be pretty high this year. There’s a good chance we see snow of some magnitude – but is it a small event or a major one? That will be dependent on both the timing of the phase (IF it occurs) and more importantly WHERE it occurs, and WHERE the H5 low closes off (if it does). A lot of IF’s at this stage in the game, but it’s better to have IF’s than flat out NO’s! Monitor the trends in Western ridge placement, the strength and progression of the southern stream short wave, and its interactions with the northern stream.

After this potential event has passed, what’s in store as we head into January? A lot of people are speculating as to whether this cold/very cold regime will break down and we go blowtorch for the month of January. I believe it’s likely we see a relaxation in temperatures compared to normal, but an all out, sustained period of above normal temperatures is unlikely up through January 15th. The AO will be climbing out of the tank, but it will not go positive. The NAO will approach neutral, but there’s a good chance we see a renewed period of blocking, probably much less severe than what we’re seeing right now, but even still, a neg NAO period.

Notice as we head out into the Day 8-10 range, there is still a decent blocking signal depicted on both the Euro and GFS.

The colder than normal temp pattern in the Northeast US shows no sign of breaking down through the remainder of December and even into early January. Beyond that time frame, it becomes less clear, but with the NAO and AO highly unlikely to surge above neutral (0), it’s also unlikely that the Northeast experiences a long warm period of any kind prior to the 15th. The correlation between the NAO/AO and Northeast temeperatures in January is very strong, so I’d expect our sensible weather to play out according to he wax/wane of those indicies. By the way, this winter so far is much more analogous to the Ninas of the 1950s and 1960s, which featured stronger negative NAO’s, as was outlined in the winter outlook this autumn.

Bottom lines to this:

1) Monitoring Christmas for a potential storm. Inland/warm track – unlikely. Suppressed/offshore more likely. Snow solution – possible, need to watch phasing of streams, western ridge placement, etc.

2) Pattern will stay cold after Christmas into early January. Moderation in temps is possible, but no sign o sustained blowtorch, pattern reveral ala the dreaded one of 1989-1990.

3) Slightly negative NAO/AO through mid January? Blocking has been a persistent element this winter thanks in large part to the low solar constant. There’s a good chance we remain cold in the Northeast through mid month.